Catalysts are used to increase the rate of chemical conversion of starting materials to products. Typically a catalyst lowers the energy of the highest energy reaction intermediate, termed the transition state intermediate. Since the rate of conversion typically increases with decreasing energy of the transition state, a catalyst speeds the conversion.
A major effort of catalysis is the pursuit of catalysts having improved activity without increasing cost. Many catalytically active metals, such as precious metals, are expensive. Thus, optimizing the usage of active materials is a major aspect of catalyst optimization.
Catalysis has been concerned with small particles for a long time. The initial incentive to reduce the size of the particles of active components was to maximize the specific cost per function. Recent investigations have focused on applications of nanotechnology in catalysis.
Studies of the use of nanoparticles for catalysis have revealed a nanocatalytic enhancement. Nanocatalytic enhancement is an increase in activity of a catalyst of nanoscale size with respect to a reference catalytst of the same material(s) at larger than nanoscale size. In nanocatalysis, the functional dimension of a chemical bond in the substrate molecule of the starting material or of the product has a typical length on the order of angstroms (Å), whereas a catalytic nanoparticle, with a size on the order of nanometers (nm), is typically more than 10 times as large. It is believed that the small ratio of the nanoparticle size to the substrate molecule size leads to the nanocatalytic enhancement.
The nanocatalytic enhancement is understood in more detail as follows. Catalytic phenomena arise from the intimate interconnection of processes between the substrate molecule and the active sites of the catalyst with solid-state chemical processes, which are brought about by the interaction of the whole catalyst material with the reactants and products. The combination of surface-science model experiments and chemical kinetics has shaped the image of a dynamic catalyst. A dynamic catalyst has a function governed by factors that are locally and temporally dependent on the catalyst function. These factors include surface structure of the catalyst, gas-phase chemical composition of the starting material and products, and the interaction of the surface structure and gas-phase chemical composition. It is believed that a reason for the nanoeffect in catalytic applications is the ability of nanostructured matter to occur in non-equilibrium modifications that are metastable under reaction conditions. It is further believed that the unique atomic-level geometries that emerge from nanoparticles enable orbital overlap of electronic densities of the catalyst with the reactant and product adsorbates that are not otherwise accessible, and they effectively lower the energy of the transition state intermediate.
Nanoparticle catalysts typically have a ball shape, or have other geometries that are essentially nanoscopic in three dimensions. Despite nanocatalytic enhancements to catalyst activity, a disadvantage of three-dimensional nanoscopic nanoparticles as stand-alone catalysts is the difficulty of handling the catalysts. Thus, there remains a need for an improved catalyst particle structure that combines the property of nanocatalytic performance with ease of handling.